In an information input and output apparatus for recording information into a recording medium and reading information from the recording medium by use of light, there is a disk-like recording medium on which concentric or spiral tracks are provided to form recording pits thereon, thereby recording binary information into the recording medium in accordance with the presence or non-presence of the recorded pit. The recorded pit is formed on the track by projecting a minute light beam spot obtained in accordance with the focusing of light radiated from a light source. Information thus recorded in the recording medium is read from the recording medium by supplying a minute light beam spot to the track and receiving light reflected from the track, thereby detecting the presence or non-presence of the recording pit. For the purpose of recording information into a recording medium and reading information from the recording medium, a typical type of an optical head apparatus comprises a light source for radiating light of a predetermined wavelength, a focusing lens for focusing the light on the recording medium, an optical system for separating light reflected on the recording medium from a path of the light directly radiated from the light source, and an optical detector for detecting the light returned from the recording medium. Such an optical head apparatus is described on pages 483 to 489 of "Applied Optics, Vol. 25, No. 4, Feb. 15, 1986".
In the optical head apparatus, the smaller the diameter of a minute light beam spot focused on the recording medium the better it is for the purpose of increasing recorded density. The diameter of the minute light beam spot depends on the wavelength ".lambda." of light radiated from the light source, and a numerical aperture "NA" of the focusing lens, such that it is proportional to ".lambda./NA". Therefore, the development of an optical head apparatus has been promoted in direction that the wavelength .lambda. is short, and the numerical aperture NA is large, so that the diameter of the minute light beam spot becomes small.
Another optical head apparatus is described on pages 553 to 557 of "Journal of the Optical Society of America, Vol. 39, No. 7, July 1949" in which a theoretical method is studied to reduce the diameter of a minute light beam spot on a recording medium. In the optical head apparatus, the light intensity of a central portion (main lobe) of focused light is lowered in light incident cross-section of a focusing lens relatively to that of a peripheral portion (side lobe) of the focused light, so that a minute light beam spot having a diameter less than the diffraction limit is obtained. This is known as the super-resoluved beam spot.
According to the former optical head apparatus, however, there is the disadvantage that the diameter of the minute light beam spot projected on the recording medium can not be smaller than the diffraction limit. Therefore, there is a limitation in increasing a recorded density of the recording medium.
According to the latter optical head apparatus, there is the disadvantage that, although the central portion (main lobe) of the focused light can be smaller in its diameter than the diffraction limit, the light intensity of the peripheral portion (side lobe) of the focused light increases. Therefore, the noise increases in signals read from the recording medium to deteriorate a read-out characteristic, where the light intensity of the peripheral portion (side lobe) of the focused light is increased.
There is another disadvantage that a light source for radiating light of high output must be provided, because the central portion (main lobe) of the focused light is interrupted to transmit to the recording medium by a light shielding member, thereby resulting in the reduction of light utilizing factor.